Field of the Invention
The present invention relates to a method of manufacturing an anti-reflective (AR) coating for a photovoltaic cell, and more particularly, to a method of manufacturing an AR coating having a moth-eye structure which can prevent surface reflection and a photovoltaic cell including the same AR coating.
Description of Related Art
A photovoltaic cell is a device that generates electrical energy by absorbing photons from the sun. FIG. 1 schematically shows a known photovoltaic cell structure. In a two-terminal junction device such as a p-n diode or a Schottky diode, which is a typical photovoltaic cell topology, electricity is generated as follows: Photons produce electron-hole pairs (excitons), which must be spontaneously divided toward both ends of the device. Before producing excitons, photons always experience blockage at the interface between air (refractive index n=1) and a window layer (n>1) of the device due to different refractive indices of the air and the window layer. This phenomenon of photon blockage referred to as surface reflection is one of well-known light loss factors in solar energy harvesting.
In order to reduce such surface reflection, as shown in FIG. 1, an anti-reflective (AR) coating is formed. At the same time, many strategies for reducing the reflection of light on the surface of a photovoltaic cell, such as deposition of a thin film having an intermediate refractive index, micro-texturing, or induction of surface plasmonic scattering using metallic nanoparticles, were proposed. In 1967, Bernard discovered the moth's cornea in which conical protuberances are uniformly arrayed. This structure was suggested as a final solution to prevent surface reflection over the whole solar radiation spectrum. The optical actions of the corneal nipple not only reduce the reflectance of the facet lens surface, but also increase the photosensitivity of the facet lens surface. The height of the nipple was proved a more essential factor than the function of the width of the nipple. The top width of the nipple was proved to be more effective than the bottom width of the nipple when reducing the reflectance.
Recently, as methods for realizing an AR structure having a large area moth-eye structure on a photovoltaic cell, top-down processes such as photolithography, nano-imprinting lithography and nano-laser interference lithography were reported. However, none of these top-down processes are successful unless the surface of a substrate is flattened, because it is difficult to coat a non-flattened surface with a lithography medium such as a photo-reactive layer. Potential materials for a thin-film photovoltaic cell include CuInGaSe2 (CIGS), a-Si and CdTe, in which a CIGS photovoltaic cell achieved the highest conversion efficiency (up to about 20%) due to its excellent light absorbance. It is important to increase the grain size of the CIGS layer when it is intended to achieve a high efficiency in the CIGS photovoltaic cell because a reduction in the grain boundary area also leads to a reduction in electron-hole recombination at the grain boundary. Unfortunately, as the grain size of the CIGS layer increases, the surface roughness condition makes it difficult to perform the top-down process. Accordingly, there has been no report on the application of an AR coating having a moth-eye structure to the CIGS photovoltaic cell.